High instep true to size for Alden Barrie last?

The normal advice for Alden's barrie last (and clark's desert boots) is to size down half a size, but my feet have high instep, so I wear my regular size on clarks. Would the same logic apply for the barrie last?

For reference, a 9E in shell perforated cap toe oxford fits comfortably and a 8D in shell blucher fits as well, but tighter (both are on the Alden Snap last). I can also wear RL (crocket jones) McCallum cordovan chukka boots in 9D, although there is some room in the shoe on all sides.